The surfaces of walls of coal in coal mines becomes unstable through oxidation which results in spalling exposing further coal to air. In addition, pressure from strata above the mine causes such walls to slip and fail.
Walls in coal mines are usually reinforced with dowels of wood and/or fibreglass reinforced plastic held in place by resin. By contrast roofs in such mines are reinforced by tensioned steel rock-bolts. Such steel bolts are not suitable to be used in the walls of coal mines as the walls themselves being coal are eventually removed by mining machinery. Steel dowels would damage the cutting heads of the mining machines. Wood and fibreglass reinforced plastic dowels cut along with the coal without damaging the cutting heads.
Whilst it would be advantageous to tension dowels in mine walls and roofs to improve resistance of such walls and roofs to spalling, slipping and failure, it has not been practicable with the materials used for such purposes as machining fibreglass reinforced plastic dowels results in loss of strength such that threaded dowels fail at unacceptable low tensions as do machined wooden dowels. As indicated above, steel dowels would interfere with normal mining practice.
A further disadvantage of steel rock-bolts is their limited life due to their susceptibility to corrosion in the water/acid environments of excavations such as tunnels and mines.
There is a need for a tensionable dowel which can be cut by mining machinery without damaging cutting heads and which would be resistant to corrosion.
Another disadvantage of steel rock-bolts used in the roof of mines is that they are prone to failure without warning and such failure is often accompanied by collapse of the roofs which they support.
Thus a need also exists for rock-bolts and tensionable dowels which are able to indicate increasing pressures in mine strata to give early warning of a potential collapse.